Chlorpromazine and its analogs are widely used in the therapy of psychiatric patients and also in experimental pharmacologic studies. Available methods for the determination of chlorpromazine in plasma are somewhat laborious in that they require extraction followed by fluorescent labeling using dansyl chloride, thin layer chromatography and spectroflorometry (Forrest et al., Agressologie 12, 127 (1970); Kaul et al., Nature 226, 372 (1970)); gas-liquid chromatography (Curry, Anal. Chem. 40, 1251 (1968)) or labeled derivative formation (Efron et al., Psychopharmacologia 19, 207 (1971)).